Hole formers, knock-out molds, and mandrels are well known in concrete casting, being utilized to accurately define and position an opening within a cast concrete structure, such as a manhole casting or drainage box. Pipes can then be mortared into the openings created by the hole formers and mandrels. Typical cast members are buried underground at depths reaching 30 feet. At these depths, accurate hole placement within the cast member becomes crucial in order to avoid costly retrenching and realignment of pipes with the cast member.
Traditionally, hole formers, mandrels and knock-out molds have been positioned in a concrete casting utilizing one of two methods. When casting the concrete, the hole former, mandrel or knock-out mold (collectively referred to as “hole formers” hereinafter) is placed so that the hole former's end cap is adjacent to at least one wall of the mold form. The first method calls for the hole former to then be attached to the mold form by drilling holes through the end cap of the hole former, into the mold form. A wire is then passed through the corresponding holes in order to attach the hole former to the inner mold and hold the hole former in place during the casting process.
However, different concrete forms often require holes in very different locations. This results in a myriad of holes that have to be drilled into the inner mold in order to accommodate and accurately place a hole in the concrete casting. This excessive drilling weakens the structural stability of the inner mold, which can result in chipped edges around unused holes. Additionally, the excess holes that are not being utilized to position a hole former become repositories for casting material, which makes removal of the casting from the inner mold more difficult. This can easily result in damaged and broken castings, thus increasing production time and costs.
Moreover, hole formers have a tendency to float in the casting medium. Wires that are used to attach the hole former to the inner mold often allow a shifting of the hole former, so that the final placement of the hole in the casting does not correspond with the precast placement. Holes in the casting therefore do not accurately align with the pipes to be attached.
The second method, of attachment involves utilizing a removable hanger that attaches to the top of the mold form. While this arrangement does not damage the mold form in ways similar to the use of bailing wire described above, it does suffer similar drawbacks in the shifting of the hole former during the casting process.
During the casting process, the mold form is often vibrated to remove air pockets from the cast material. This vibration often has the unfortunate effect of shifting the hole former from its desired location. While traditional hangers may retain the vertical alignment of the hole former during this vibration, horizontal or radial movement of the hole former is not as easily controlled.
Some manufacturers have tried to overcome the use of the hanging apparatus by securing the hole former in place with a centrally situated magnet. These devices also overcome problem of damaging the mold form, but are still subject to shifting of the hole former due to the combined buoyant and vibrational forces that are present during the casting process. This problem is compounded when the magnet is to be used with a manhole casting, where the mold form has a radial surface. The planar surface of the magnets centrally disposed in the hole former are not able to fully engage the radial surface of the manhole casting, resulting in a weakened attraction between the magnet and the mold form. Without experiencing the full force of the magnets, hole formers are much more easily displaced from their desired position during the casting process.
It would therefore be desirable to be able to accurately place and secure a hole former during the casting process that would not degrade the integrity of the mold form. Additionally, it would be beneficial to securely maintain the placement of the hole former during the casting process, overcoming any buoyant forces that may displace the hole former from its intended positioning. Finally, it would be beneficial to have a means of securing a hole former that would work equally well on both flat and radial surfaces.